An unacceptable degree of porosity was identified in several closure welds on stainless steel containers for plutonium-bearing materials. The pores developed in the weld tie-in region due to gas trapped by the weld pool during the closure process. This paper describes the efforts to trace the root cause of the porosity to the geometric conditions of the weld joint and establish corrective actions to minimize such porosity.
The clapper in a 250 mm diam disk valve (made from ASTM A36 steel, stress relieved and cadmium plated) fractured at the welded joint between the clapper and a 20 mm diam support rod (also made of same material). The valve contained a stream of gas consisting of 55% H2S, 39% CO2, 5% H2, and 1% hydrocarbons at 40 deg C and 55 kPa during operation. Voids on the fracture surface and evidence of incomplete weld penetration were revealed by examination. Brittle fracture was indicated by the overall appearance through some fatigue beach marks were observed. Very narrow bands of high hardness were revealed at the edges of the weld metal. It was revealed by chemical analysis of this band that a stainless steel filler metal had been used which produced mixed composition at the weld boundaries. The plating material was revealed to be nickel by chemical analysis. It was concluded that clapper failed by fatigue and brittle fracture because it was welded with an incorrect filler metal. A clapper assembly was welded with a low-carbon steel filler metal, then cadmium plated.